Fast-path for uncontended merge

Merge

```
../gradlew benchmarks '-Pjmh=-f 1 -tu s -bm thrpt -wi 5 -i 5 -r 2 -prof GC rx.usecases.PerfTransforms.flatMap.*'
```

Before ...

```
Benchmark                                       (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.flatMapInt                        1  thrpt         5   712336.404    19393.950    ops/s
r.u.PerfTransforms.flatMapInt                     1024  thrpt         5       33.312        1.814    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake        1  thrpt         5   618673.667    21798.483    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake     1024  thrpt         5        0.628        0.072    ops/s
```

After ...

```
Benchmark                                       (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.flatMapInt                        1  thrpt         5   817098.263    26244.709    ops/s
r.u.PerfTransforms.flatMapInt                     1024  thrpt         5      933.212       13.307    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake        1  thrpt         5   691744.762    31131.700    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake     1024  thrpt         5       70.242        3.444    ops/s
```

... but 0.19 master is still far better on the first 3 tests:

```
Benchmark                                       (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.flatMapInt                        1  thrpt         5  3372557.564   131729.592    ops/s
r.u.PerfTransforms.flatMapInt                     1024  thrpt         5     6713.975      159.005    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake        1  thrpt         5  2231392.499    50190.297    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake     1024  thrpt         5       16.437        0.413    ops/s
```